Assay Methods: Preoperative levels of total serum calcium (reference range, 8.6-10.2 mg/dL) will be determined by auto-analyzer (Modular P: Roche Diagnostics, Germany). The serum Calcium level will be corrected with the serum albumin level. The serum intact PTH (reference range, 12-55 ng/L)and 25-hydroxyvitamin D (11.1-42.9 ng/ml) will be measured using immunochemiluminiscence (ELECSYS-2010: Roche Diagnostics, Germany).